Heat dissipation system with hygroscopic working fluid

ABSTRACT

A system and method for transferring heat from a process source and dissipating it to the ambient atmosphere. The system uses a low-volatility, hygroscopic working fluid to reject thermal energy directly to ambient air. Direct-contact heat exchange allows for the creation of large interfacial surface areas for effective heat transfer. Heat transfer is further enhanced by water vapor pressure gradients present between the equilibrium moisture content of the working fluid and the ambient air. Cyclic absorption and evaporation of atmospheric moisture dampens variations in cooling capacity because of ambient temperature changes. The low-volatility and hygroscopic nature of the working fluid prevents complete evaporation of the fluid and a net consumption of water.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. Provisional Patent ApplicationSer. No. 61/345,864 filed May 18, 2010, which is incorporated herein inits entirety by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under CooperativeAgreement No. DE-FC26-08NT43291 entitled “EERC-DOE Joint Program onResearch and Development for Fossil Energy-Related Resources,” awardedby the U.S. Department of Energy (DOE). The government has certainrights in the invention.

FIELD OF THE INVENTION

This invention relates to the dissipation of degraded thermal energy toambient air.

BACKGROUND OF THE INVENTION

Cooling and thermal energy dissipation are universal tasks in industry.Common heat rejection processes include steam condensation inthermoelectric power plants, refrigerant condensation inair-conditioning and refrigeration equipment, and process cooling duringchemical manufacturing. In the case of power plants and refrigerationsystems, it is desired to dissipate thermal energy at the lowestpossible temperature and as close as possible to the operatingenvironment for optimum energy efficiency.

Where the local environment has a suitable low-temperature sink, e.g., ariver, sea, or lake, cooling water can be extracted directly. However,few of these opportunities for once-through cooling are expected to beavailable in the future because of competition for water sources andrecognition of their impact on the environment. In the absence of asuitable coolant source, the only common thermal sink available at alllocations is ambient air. Both sensible and latent heat transfer arecurrently used to reject heat to the air. In sensible cooling, air isused directly as the coolant, and it is used to cool one side of theprocess heat exchanger. For latent cooling, liquid water is used as thecoolant, and it is then itself cooled by partial evaporation in acooling tower. The thermal energy is transferred to the ambient air inthe form of evaporated water vapor, with minimal temperature rise of theair.

These technologies are used routinely in industry, but each one hasdistinct drawbacks. In the sensible cooling case, air is an inferiorcoolant compared to liquids, and the resulting efficiency of air-cooledprocesses can be poor. The air-side heat-transfer coefficient isinvariably much lower than liquid-cooled heat exchangers or condensationprocesses and, therefore, requires a large heat exchange surface areafor good performance. In addition to larger surface area requirements,air-cooled heat exchangers approach the ambient dry-bulb temperature,which can vary 30° to 40° F. over the course of a day and can hindercooling capacity during the hottest hours of the day. Air-cooled systemdesign is typically a compromise between process efficiency and heatexchanger cost. Choosing the lowest initial cost option can havenegative energy consumption implications for the life of the system.

With latent heat dissipation, the cooling efficiency is much higher, andthe heat rejection temperature is more consistent throughout the courseof a day since a wet cooling tower will approach the ambient dew pointtemperature instead of the oscillatory dry-bulb temperature. The keydrawback of this approach is the associated water consumption, which inmany areas is becoming a limiting resource. Obtaining sufficient waterrights for wet cooling system operation delays plant permitting, limitssite selection, and creates a highly visible vulnerability for opponentsof new development.

Improvements have been proposed to these basic cooling systems. Asignificant effort has gone into hybrid cooling concepts that augmentair-cooled condensers with evaporative cooling during the hottest partsof the day. These systems can use less water compared to complete latentcooling, but the performance benefit is directly related to the amountof water-based augmentation, so these systems do not solve theunderlying issue of water consumption. Despite the fact that meeting thecooling needs of industrial processes is a fundamental engineering task,significant improvements are still desired, primarily the elimination ofwater consumption while simultaneously maintaining high-efficiencycooling at reasonable cost.

In summary, there is a need for improved heat dissipation technologyrelative to current methods. Sensible cooling with air is costly becauseof the vast heat exchange surface area required and because itsheat-transfer performance is handicapped during the hottest ambienttemperatures. Latent or evaporative cooling has preferred coolingperformance, but it consumes large quantities of water which is alimited resource in some locations.

SUMMARY OF THE INVENTION

A heat dissipation system apparatus and method of operation usinghygroscopic working fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the heat dissipation system according to oneembodiment of the present invention.

FIG. 2A is a chart depicting the input temperature conditions used tocalculate the dynamic response of one embodiment of the presentinvention.

FIG. 2B is a chart depicting the calculated components of heat transferof the present invention in response to the cyclical input temperatureprofile of FIG. 2A.

FIG. 3 is a schematic of a cross-flow air contactor depicting analternate embodiment of the present invention.

FIG. 4 is a cross-sectional detail of one of the tube headers shown inthe air contactor of FIG. 3.

FIG. 5A is a schematic of a falling-film process heat exchangerdepicting an alternate embodiment of the present invention.

FIG. 5B is a section view of the process heat exchanger in FIG. 5A asviewed from the indicated section line.

FIG. 6 is a schematic of an alternate embodiment of the presentinvention incorporating a falling-film process heat exchanger toprecondition the air contactor inlet air.

FIG. 7 is a schematic of an alternate embodiment of the presentinvention incorporating the air contactor to precondition a falling-filmprocess heat exchanger.

FIG. 8 is a schematic of an alternate embodiment of the presentinvention incorporating alternate means to increase the moisture contentof the working fluid.

DETAILED DESCRIPTION OF THE INVENTION

The heat dissipation system described herein circulates a hygroscopicworking fluid to transfer heat from a process requiring cooling directlyto the ambient air. The hygroscopic fluid is in liquid phase atconditions in which it is at thermal and vapor pressure equilibrium withthe expected local ambient conditions. The fluid is composed of asolution of a hygroscopic substance and water. In one embodiment, thehygroscopic substance itself should have a very low vapor pressurecompared to water in order to prevent significant loss of thehygroscopic component during cycle operation. The hygroscopic componentcan be a pure substance or a mixture of substances selected fromcompounds known to attract moisture vapor and form liquid solutions withwater that have reduced water vapor pressures. The hygroscopic componentincludes all materials currently employed for desiccation operations ordehumidifying operations including hygroscopic inorganic salts, such asLiCl, LiBr, CaCl₂, ZnCl₂; hygroscopic organic compounds, such asethylene glycol, propylene glycol, triethylene glycol; or inorganicacids, such as H₂SO₄ and the like.

Thermal energy is removed from the process in a suitable heat exchangerhaving one side thereof, the flow of process fluid, and on the otherside thereof, the flow of hygroscopic working fluid coolant. This heatexchanger can take the form of any well-known heat exchange device,including shell-and-tube heat exchangers, plate-and-frame heatexchangers, or falling-film heat exchangers. The process fluid beingcooled includes a single-phase fluid, liquid, or gas or can be a fluidundergoing phase change, e.g., condensation of a vapor into a liquid.Consequently, the thermal load presented by the process fluid can besensible, i.e., with a temperature change, or latent which isisothermal. Flowing through the other side of the heat exchange device,the hygroscopic working fluid coolant can remove heat sensibly, such asin a sealed device with no vapor space, or it can provide a combinationof sensible and latent heat removal if partial evaporation of themoisture in solution is allowed, such as in the film side of afalling-film heat exchanger.

After thermal energy has been transferred from the process fluid to thehygroscopic working fluid, the hygroscopic fluid is circulated to anair-contacting device where it is exposed directly to ambient air forheat dissipation. The contacting device is constructed in such a way asto generate a large amount of interfacial surface area between thesolution and air. Any well-known method may be used to generate theinterfacial area, such as by including a direct spray of the liquid intothe air, a flow of solution distributed over random packing's, or afalling film of liquid solution clown a structured surface. Flow of theair and solution streams can be conducted in the most advantageous wayfor a particular situation, such as countercurrent where the solutionmay be flowing down by gravity and the air is flowing up, cross-flowwhere the flow of solution is in an orthogonal direction to airflow,cocurrent where the solution and air travel in the same direction, orany combination of these flow types.

Heat- and mass-transfer processes inside the air contactor are enhancedby convective movement of air through the contactor. Convective flow maybe achieved by several different means or a combination of suchdifferent means. The first means for convective airflow is throughnatural convection mechanisms such as by the buoyancy difference betweenwarmed air inside the contactor and the cooler and the surroundingambient air. This effect would naturally circulate convective airflowthrough a suitably designed chamber in which the air is being heated bythe warmed solution. Another means for convective airflow includes theforced flow of air generated by a fan or blower. A further convectiveairflow means includes inducing airflow using momentum transfer from ajet of solution pumped out at sufficient mass flow rate and velocity.

Inside the air contactor, an interrelated process of heat and masstransfer occurs between the hygroscopic solution and the airflow thatultimately results in the transfer of thermal energy from the solutionto the air. When the air and solution are in contact, they will exchangemoisture mass and thermal energy in order to approach equilibrium, whichfor a desiccant liquid and its surrounding atmosphere requires a matchof temperature and water vapor pressure. Since the solution's vaporpressure is partially dependent on temperature, the condition is oftenreached where the solution has rapidly reached its equivalent dew pointtemperature by primarily latent heat transfer (to match the ambientvapor pressure), and then further evaporation or condensation is limitedby the slower process of sensible heat transfer between the air andsolution (to match the ambient temperature). The induced mass transferrequired to equilibrate vapor pressure is an added gradient thatenhances sensible heat transfer.

The net amount of heat and mass transfer within the air contactor isdependent on the specific design of the air contactor and the inletconditions of the hygroscopic solution and the ambient air. However, thepossible outcomes as solution passes through the contactor includesituations where the solution can experience a net loss of moisture (aportion of the thermal energy contained in the solution is released aslatent heat during moisture evaporation; this increases the humiditycontent of the airflow), the solution can experience a net gain inmoisture content (such occurs when the vapor pressure in the air ishigher than in the solution, and moisture is absorbed by the solutionhaving the latent heat of absorption released into the solution andbeing transferred sensibly to the air), and the solution is in a steadystate where no net moisture change occurs (any evaporation beingcounterbalanced by an equivalent amount of reabsorption, or vice versa).Even in this last instance where there is no net moisture change, thecounterbalancing processes of evaporation and reabsorption still havethe potential to enhance sensible heat transfer by altering the workingfluid temperature and, thus, the sensible heat transfer gradient.

After passing through the air contactor, the solution has releasedthermal energy to the ambient air either through sensible heat transferalone or by a combination of sensible and latent heat transfer (alongwith any concomitant moisture content change). The solution is thencollected in a reservoir, the size of which will be selected to offerthe best dynamic performance of the overall cooling system for a givenenvironmental location and thermal load profile. It can be appreciatedthat the reservoir can alter the time constant of the cooling system inresponse to dynamic changes in environmental conditions. For example,moisture absorption in the ambient atmosphere will be most encouragedduring the night and early morning hours, typically when diurnaltemperatures are at a minimum, and an excess of moisture may becollected. On the other extreme, moisture evaporation in the ambientatmosphere will be most prevalent during the afternoon when diurnaltemperatures have peaked, and there could be a net loss of solutionmoisture content. Therefore, for a continuously operating system in theambient atmosphere, the reservoir and its method of operation can beselected so as to optimize the storage of excess moisture gained duringthe night so that it can be evaporated during the next afternoon, tomaintain cooling capacity.

The reservoir itself can be a single mixed tank where the averageproperties of the solution are maintained. The reservoir also includes astratified tank or a series of separate tanks intended to preserve thedistribution of water collection throughout a diurnal cycle so collectedwater can be metered out to provide maximum benefit.

The present heat dissipation system includes the use of a hygroscopicworking fluid to remove thermal energy from a process stream anddissipate it to the atmosphere by direct contact of the working fluidand ambient air. This enables several features that are highlybeneficial for heat dissipation systems, including 1) using the workingfluid to couple the concentrated heat-transfer flux in the process heatexchanger to the lower-density heat-transfer flux of ambient air heatdissipation, 2) allowing for large interfacial surface areas between theworking fluid and ambient air, 3) enhancing working fluid-airheat-transfer rates with simultaneous mass transfer, and 4) moderatingdaily temperature fluctuations by cyclically absorbing and releasingmoisture vapor from and to the air.

Referring to drawing FIG. 1, one embodiment of heat dissipation system10 is illustrated using a hygroscopic working fluid 1 in storagereservoir 2 drawn by pump 3 and circulated through process heatexchanger 4. In the process heat exchanger, the hygroscopic workingfluid removes thermal energy from the process fluid that enters hot-sideinlet 5 and exits through hot-side outlet 6. The process fluid can be asingle phase (gas or liquid) that requires sensible cooling or it couldbe a two-phase fluid that undergoes a phase change in the process heatexchanger, e.g., condensation of a vapor into a liquid.

After absorbing thermal energy in process heat exchanger 4, thehygroscopic working fluid is routed to distribution nozzles 7 where itis exposed in a countercurrent fashion to air flowing through aircontactor 8. Ambient airflow through the air contactor in drawing FIG. 1is from bottom ambient air inlet 9 vertically to top air outlet 11 andis assisted by the buoyancy of the heated air and by powered fan 13.Distributed working fluid 12 in the air contactor flows down,countercurrent to the airflow by the pull of gravity. At the bottom ofair contactor 8, the working fluid is separated from the inlet airflowand is returned to stored solution 1 in reservoir 2.

In air contactor 8, both thermal energy and moisture are exchangedbetween the hygroscopic working fluid and the airflow, but because ofthe moisture retention characteristics of the hygroscopic solution,complete evaporation of the working fluid is prevented.

If the heat dissipation system 10 is operated continuously withunchanging ambient air temperature, ambient humidity, and a constantthermal load in process heat exchanger 4, a steady-state temperature andconcentration profile will be achieved in air contactor 8. Under theseconditions, the net moisture content of stored working fluid 1 willremain unchanged. That is not to say that no moisture is exchangedbetween distributed working fluid 12 and the airflow in air contactor 8,but it is an indication that any moisture evaporated from working fluid12 is reabsorbed from the ambient airflow before the solution isreturned to reservoir 2.

However, prior to reaching the aforementioned steady-state condition andduring times of changing ambient conditions, heat dissipation system 10may operate with a net loss or gain of moisture content in working fluid1. When operating with a net loss of working fluid moisture, theequivalent component of latent thermal energy contributes to the overallcooling capacity of the heat dissipation system 10. In this case, theadditional cooling capacity is embodied by the increased moisture vaporcontent of airflow 11 exiting air contactor 8.

Conversely, when operating with a net gain of working fluid moisturecontent, the equivalent component of latent thermal energy must beabsorbed by the working fluid and dissipated to the airflow by sensibleheat transfer. In this case, the overall cooling capacity of the systemis diminished by the additional latent thermal energy released to theworking fluid. Airflow 11 exiting air contactor 8 will now have areduced moisture content compared to inlet ambient air 9.

Another embodiment of heat dissipation system 10 illustrated in drawingFIG. 1 uses the supplementation of the relative humidity of inletambient air 9 with supplemental gas stream 40 entering throughsupplemental gas stream inlet 41. When used, gas stream 40 can be anygas flow containing sufficient moisture vapor including ambient air intowhich water has been evaporated either by misting or spraying, anexhaust stream from a drying process, an exhaust stream of high-humidityair displaced during ventilation of conditioned indoor spaces, anexhaust stream from a wet evaporative cooling tower, or a flue gasstream from a combustion source and the associated flue gas treatmentsystems. The benefit of using supplemental gas stream 40 is to enhancethe humidity level in air contactor 8 and encourage absorption ofmoisture into dispersed working fluid 12 in climates having low ambienthumidity. It is also understood that supplemental gas stream 40 wouldonly be active when moisture absorption is needed to provide a netbenefit to cyclic cooling capacity, e.g., where the absorbed moisturewould be evaporated during a subsequent time of peak cooling demand orwhen supplemental humidity is needed to prevent excessive moisture lossfrom the working fluid.

With the operation of the heat dissipation system 10 described hereinand the effects of net moisture change set forth, the performancecharacteristics of cyclic operation can be appreciated. Illustrated indrawing FIG. 2A is a plot of the cyclic input conditions of ambient airdry-bulb temperature and dew point temperature. The cycle has a periodof 24 hours and is intended to be an idealized representation of adiurnal temperature variation. The moisture content of the air isconstant for the input data of FIG. 2A since air moisture content doesnot typically vary dramatically on a diurnal cycle.

Illustrated in drawing FIG. 2B is the calculated heat-transfer responseof the present invention corresponding to the input data of FIG. 2A. Thetwo components of heat transfer are sensible and latent, and their sumrepresents the total cooling capacity of the system. As shown in drawingFIG. 2B, the sensible component of heat transfer (Q_(sensible)) variesout of phase with the ambient temperature since sensible heat transferis directly proportional to the working fluid and the airflowtemperature difference (all other conditions remaining equal). Inpractice, a conventional air-cooled heat exchanger is limited by thisfact. In the case of a power plant steam condenser, this is the leastdesirable heat-transfer limitation since cooling capacity is at aminimum during the hottest part of the day, which frequently correspondsto periods of maximum demand for power generation.

The latent component of heat transfer illustrated in drawing FIG. 2B(Q_(latent)) is dependent on the ambient moisture content and themoisture content and temperature of the hygroscopic working fluid.According to the sign convention used in drawing FIG. 2B, when thelatent heat-transfer component is positive, evaporation is occurringwith a net loss of moisture, and the latent thermal energy is dissipatedto the ambient air; when the latent component is negative, thehygroscopic solution is absorbing moisture, and the latent energy isbeing added to the working fluid, thereby diminishing overall coolingcapacity. During the idealized diurnal cycle illustrated in drawing FIG.2A, the latent heat-transfer component illustrated in drawing FIG. 2Bindicates that moisture absorption and desorption occur alternately asthe ambient temperature reaches the cycle minimum and maximum,respectively. However, over one complete cycle, the net water transferwith the ambient is zero, i.e., the moisture absorbed during the nightequals the moisture evaporated during the next day, so there is no netwater consumption.

The net cooling capacity of the heat dissipation system 10 isillustrated in drawing FIG. 2B as the sum of the sensible and latentcomponents of heat transfer (Q_(sensible)+Q_(latent)). As illustrated,the latent component of heat transfer acts as thermal damping for theentire system by supplementing daytime cooling capacity with evaporativecooling, region E₁ illustrated in drawing FIG. 2B. This evaporative heattransfer enhances overall heat transfer by compensating for decliningsensible heat transfer during the diurnal temperature maximum, regionE₂. This is especially beneficial for cases like a power plant steamcondenser where peak conversion efficiency is needed during the hottestparts of the day.

The cost of this boost to daytime heat transfer comes at night when theabsorbed latent energy, region E₃, is released into the working fluidand must be dissipated to the airflow. During this time, the totalsystem cooling capacity is reduced by an equal amount from its potentialvalue, region E₄. However, this can be accommodated in practice sincethe nighttime ambient temperature is low and overall heat transfer isstill acceptable. For a steam power plant, the demand for peak powerproduction is also typically at a minimum at night.

Regarding air contactor configuration, direct contact of the workingfluid and surrounding air allows the creation of significant surfacearea with fewer material and resource inputs than are typically requiredfor vacuum-sealed air-cooled condensers or radiators. The solution-airinterfacial area can be generated by any means commonly employed inindustry, e.g., spray contactor, wetted packed bed (with regular orrandom packings), or a falling-film contactor.

Air contactor 8, illustrated in drawing FIG. 1, is shown to be acounterflow spray contactor. While the spray arrangement is an effectiveway to produce significant interfacial surface area, in practice suchdesigns can have undesirable entrained aerosols carried out of thecontactor by the airflow. An alternate embodiment of the air contactorto prevent entrainment is illustrated in drawing FIG. 3, which is across-flow, falling-film contactor designed to minimize dropletformation and liquid entrainment. Particulate sampling across such anexperimental device has demonstrated that there is no propensity foraerosol formation with this design.

Illustrated in drawing FIG. 3, inlet hygroscopic working fluid 14 ispumped into distribution headers at the top of contactor 16. Referringto drawing FIG. 4, which is a cross section of an individualdistribution header, working fluid 17 is pumped through distributionholes 18 perpendicular to the axis of tube header 19 where it wetsfalling-film wick 20 constructed from a suitable material such as wovenfabric, plastic matting, or metal screen. Film wick support 21 is usedto maintain the shape of each wick section. Illustrated in drawing FIG.3, distributed film 22 of the working fluid solution flows down bygravity all of the way to the surface of working fluid 23 in reservoir24. Inlet airflow 25 flows horizontally through the air contactorbetween falling-film sheets 26. In the configuration illustrated indrawing FIG. 3, heat and mass transfer take place between distributedfilm 22 of working fluid and airflow 25 between falling-film sections26. While drawing FIG. 3 illustrates a cross-flow configuration, it isunderstood that countercurrent, cocurrent, or mixed flow is alsopossible with this configuration.

Illustrated in drawing FIG. 1, the process heat exchanger 4 can assumethe form of any indirect heat exchanger known in the art such as ashell-and-tube or plate-type exchanger. One specific embodiment of theheat exchanger that is advantageous for this service is the falling-filmtype. Illustrated in drawing FIG. 5A is a schematic of alternateembodiment process heat exchanger 27. Illustrated in drawing FIG. 5B isa cross-sectional view of process heat exchanger 27 viewed along theindicated section line in drawing FIG. 5A. Referring to drawing FIG. 5B,process fluid 28 (which is being cooled) is flowing within tube 29.Along the top of tube 29, cool hygroscopic working fluid 30 isdistributed to form a film surface which flows down by gravity over theoutside of tube 29. Flowing past the falling-film assembly is airflow 31which is generated either by natural convection or by forced airflowfrom a fan or blower.

As working fluid 30 flows over the surface of tube 29, heat istransferred from process fluid 28 through the tube wall and into theworking fluid film by conduction. As the film is heated, its moisturevapor pressure rises and may rise to the point that evaporation takesplace to surrounding airflow 31, thereby dissipating thermal energy tothe airflow. Falling-film heat transfer is well known in the art as anefficient means to achieve high heat-transfer rates with lowdifferential temperatures. One preferred application for thefalling-film heat exchanger is when process fluid 28 is undergoing aphase change from vapor to liquid, as in a steam condenser, wheretemperatures are isothermal and heat flux can be high.

A further embodiment of the heat dissipation system 10 is illustrated indrawing FIG. 6. The heat dissipation system 10 incorporates thefilm-cooled process heat exchanger to condition a portion of the airflowentering air contactor 8. Illustrated in drawing FIG. 6, process heatexchanger 32 is cooled by a falling film of hygroscopic working fluidinside housing 33. Ambient air 34 is drawn into process heat exchangerhousing 33 and flows past the film-cooled heat exchanger where itreceives some quantity of evaporated moisture from the film. Thehigher-humidity airflow at 35 is conducted to inlet 36 of air contactor8 where it flows countercurrent to the spray of hygroscopic workingfluid 12. Additional ambient air may also be introduced to the inlet ofair contactor 8 through alternate opening 38.

In the embodiment illustrated in drawing FIG. 6, moisture vapor releasedfrom process heat exchanger 32 is added to the air contactor's inletairstream and thereby increases the moisture content by a finite amountabove ambient humidity levels. This effect will tend to inhibit moistureevaporation from working fluid 12 and will result in a finite increaseto the steady-state moisture content of reservoir solution 1. Theembodiment illustrated in drawing FIG. 6 may be preferred in aridenvironments and during dry weather in order to counteract excessiveevaporation of moisture from the working fluid.

A further embodiment of the heat dissipation system 10 is illustrated indrawing FIG. 7. The heat dissipation system 10 incorporates the aircontactor to condition the airflow passing the film-cooled process heatexchanger. As illustrated in drawing FIG. 7, a portion of the airflowexiting air contactor 8 at outlet 39 is conducted to the inlet ofprocess heat exchanger housing 33. This airflow then flows pastfilm-cooled process heat exchanger 32 where it receives moisture fromfilm moisture evaporation.

During high-ambient-humidity conditions when the net moisture content ofreservoir solution 1 is increasing, the air at outlet 39 will have alower moisture vapor content than ambient air 9 entering air contactor8. Therefore, some advantage will be gained by exposing film-cooledprocess heat exchanger 32 to this lower-humidity airstream from outlet39 rather than the higher-humidity ambient air. The lower-humidity airwill encourage evaporation and latent heat transfer in film-cooledprocess heat exchanger 32, and it will allow for lower film temperaturesbecause of the lower dew point associated with the lower-humidity air.The embodiment illustrated in drawing FIG. 7 may be preferred forhigh-humidity conditions since it will enhance the latent component ofheat transfer when a film-cooled process heat exchanger is used.

A further embodiment of the heat dissipation system 10 is illustrated indrawing FIG. 8. The heat dissipation system 10 uses an alternate meansfor increasing the working fluid moisture content above those that couldbe obtained by achieving equilibrium with the ambient air. The firstalternative presented in drawing FIG. 8 is to increase moisture contentof working fluid 1 directly by addition of liquid water stream 42. Inthe other alternative presented, working fluid 1 is circulated throughabsorber 43 where it is exposed to gas stream 44. Gas stream 44 hashigher moisture vapor availability compared to ambient air 9. Therefore,the working fluid that passes through absorber 43 is returned toreservoir 2 having a higher moisture content than that achievable in aircontactor 8. The source of gas stream 44 may include ambient air intowhich water has been evaporated either by misting or spraying, anexhaust stream from a drying process, an exhaust stream of high-humidityair displaced during ventilation of conditioned indoor spaces, anexhaust stream from a wet evaporative cooling tower, or a flue gasstream from a combustion source and the associated flue gas treatmentsystems. The benefit of such alternatives illustrated in drawing FIG. 8is to increase the moisture content of working fluid 1 during periods oflow heat dissipation demand, such as at night, for the purpose ofproviding additional latent cooling capacity during periods when heatdissipation demand is high.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications, and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A method for heat dissipation comprising: providing a low-volatilityhygroscopic working fluid, removing heat from a process heat exchangerto absorb thermal energy for dissipation using the low-volatilityhygroscopic working fluid, providing a working fluid-air contactor,enabling combined heat dissipation from the low-volatility hygroscopicworking fluid to the air using the fluid-air contactor, and enabling abidirectional moisture mass transfer between the low-volatilityhygroscopic working fluid and the air using the working fluid-aircontactor.
 2. The method for heat dissipation according to claim 1,wherein the hygroscopic working fluid comprises an aqueous solutionincluding at least one of sodium chloride (NaCl), calcium chloride(CaCl₂), lithium chloride (LiCl), lithium bromide (LiBr), zinc chloride(ZnCl₂), sulfuric acid (H₂SO₄), sodium hydroxide (NaOH), sodium sulfate(Na₂SO₄), potassium chloride (KCl), calcium nitrate (Ca[NO₃]₂),potassium carbonate (K₂CO₃), ammonium nitrate (NH₄NO₃), ethylene glycol,diethylene glycol, propylene glycol, triethylene glycol, dipropyleneglycol, and any combination thereof.
 3. The method for heat dissipationaccording to claim 1, wherein the process heat exchanger comprises oneof a condenser of a thermodynamic power production or a refrigerationcycle.
 4. The method for heat dissipation according to claim 1, whereinthe fluid-air contactor operates in at least one relative motionincluding countercurrent, cocurrent, or cross-flow operation.
 5. Themethod for heat dissipation according to claim 1, wherein the fluid-aircontactor is enhanced by at least one of the forced or induced draft ofambient air by a powered fan; the natural convection airflow generatedfrom buoyancy differences between heated and cooled air; and the inducedflow of air generated by the momentum transfer of sprayed working fluidinto the air.
 6. The method for heat dissipation according to claim 1,wherein said ambient airstream is supplemented with additional humidityfrom at least one of: a spray, mist, or fog of water directly into theairstream, an exhaust gas stream from a drying process, an exhaust gasstream consisting of high-humidity rejected air displaced during theventilation of conditioned indoor spaces, an exhaust airstream from awet evaporative cooling tower, and an exhaust flue gas stream from acombustion source and any associated flue gas treatment equipment. 7.The method for heat dissipation according to claim 1, wherein theoverall heat-transfer performance is enhanced by addition of moisture tothe hygroscopic working fluid using any one of: direct addition ofliquid water to the hygroscopic working fluid and absorption ofvapor-phase moisture by the working fluid from a moisture-containing gasstream outside of the process air contactor, where themoisture-containing gas stream could include ambient air into whichwater has been evaporated by spraying or misting, flue gas from acombustion source and its associated flue gas treatment equipment,exhaust gas from a drying process, rejected high-humidity air displacedduring ventilation of conditioned indoor air, or the exhaust airstreamfrom a wet evaporative cooling tower.
 8. The method for heat dissipationaccording to claim 1, wherein the process heat exchanger is cooled by aflowing film of said hygroscopic working fluid enabling both sensibleand latent heat transfer to occur during thermal energy absorption fromthe process fluid.
 9. The method for heat dissipation according to claim8, wherein the process heat exchanger is placed at the inlet to said aircontactor for raising inlet airflow humidity levels.
 10. The method forheat dissipation according to claim 8, wherein the process heatexchanger is placed at the outlet of said air contactor for receivingair dehumidified with respect to the ambient air atmosphere.
 11. A heatdissipation method comprising: removing heat from a process heatexchanger absorbing thermal energy using a low-volatility hygroscopicworking fluid, enabling combined heat dissipation from thelow-volatility hygroscopic working fluid to the air using a fluid-aircontactor, and enabling a bidirectional moisture mass transfer betweenthe low-volatility hygroscopic working fluid and the air using theworking fluid-air contactor.
 12. The method for heat dissipationaccording to claim 11, wherein the hygroscopic working fluid comprisesan aqueous solution including at least one of sodium chloride (NaCl),calcium chloride (CaCl₂), lithium chloride (LiCl), lithium bromide(LiBr), zinc chloride (ZnCl₂), sulfuric acid (H₂SO₄), sodium hydroxide(NaOH), sodium sulfate (Na₂SO₄), potassium chloride (KCl), calciumnitrate (Ca[NO₃]₂), potassium carbonate (K₂CO₃), ammonium nitrate(NH₄NO₃), ethylene glycol, diethylene glycol, propylene glycol,triethylene glycol, dipropylene glycol, and any combination thereof. 13.The method for heat dissipation according to claim 11, wherein theprocess heat exchanger comprises one of a condenser of a thermodynamicpower production or a refrigeration cycle.
 14. The method for heatdissipation according to claim 11, wherein the fluid-air contactoroperates in at least one relative motion including countercurrent,cocurrent, or cross-flow operation.
 15. The method for heat dissipationaccording to claim 11, wherein the fluid-air contactor is enhanced by atleast one of the forced or induced draft of ambient air by a poweredfan; the natural convection airflow generated from buoyancy differencesbetween heated and cooled air; and the induced flow of air generated bythe momentum transfer of sprayed working fluid into the air.
 16. Themethod for heat dissipation according to claim 11, wherein said ambientairstream is supplemented with additional humidity from at least one ofa spray, mist, or fog of water directly into the airstream, an exhaustgas stream from a drying process, an exhaust gas stream consisting ofhigh-humidity rejected air displaced during the ventilation ofconditioned indoor spaces, an exhaust airstream from a wet evaporativecooling tower, and an exhaust flue gas stream from a combustion sourceand any associated flue gas treatment equipment.
 17. The method for heatdissipation according to claim 11, wherein the overall heat-transferperformance is enhanced by addition of moisture to the hygroscopicworking fluid using at least one of direct addition of liquid water tothe hygroscopic working fluid and absorption of vapor-phase moisture bythe working fluid from a moisture-containing gas stream outside of theprocess air contactor, where the moisture-containing gas stream couldinclude ambient air into which water has been evaporated by spraying ormisting, flue gas from a combustion source and its associated flue gastreatment equipment, exhaust gas from a drying process, rejectedhigh-humidity air displaced during ventilation of conditioned indoorair, or the exhaust airstream from a wet evaporative cooling tower. 18.The method for heat dissipation according to claim 11, wherein theprocess heat exchanger is cooled by a flowing film of said hygroscopicworking fluid enabling both sensible and latent heat transfer to occurduring thermal energy absorption from the process fluid.
 19. The methodfor heat dissipation according to claim 18, wherein the process heatexchanger is placed at the inlet to said air contactor for raising inletairflow humidity levels.
 20. The method for heat dissipation accordingto claim 18, wherein the process heat exchanger is placed at the outletof said air contactor for receiving air dehumidified with respect to theambient air atmosphere.